1. Field of the Invention
The present invention relates to an exhaust re-circulation control device for internal combustion engines which re-circulates exhaust gas to an air intake of an engine in order to reduce NOx emissions and, particularly to an exhaust re-circulation control device which contributes to efficient reduction of NOx in low-temperature idling range of a gasoline.
2. Description of Related Art
As is well known, exhaust re-circulation control devices are devices which extract a portion of the exhaust gas emissions from the exhaust system of an internal combustion engine and control it to the appropriate temperature, timing, quantity of flowing, etc., for re-circulating the exhaust gas emissions back to the air intake system. Thus, the combustion temperature of mixture is lowered, and finally NOx emissions which constitute a toxic substance (NO and NO2 of nitrogen oxides) produced as a byproduct of the combustion is suppressed.
Such exhaust gas re-circulation control has been conventionally prohibited during so-called idling, when the accelerator is closed, so as to prevent deterioration of driveability and engine stalling.
However, in recent years, accompanying tightening of exhaust gas emission regulations, exhaust gas re-circulation control has been performed as part of studies for the further reduction of NOx exhaust gas emissions.
Conventionally, as devices which perform exhaust re-circulation control even during engine idling, the devices described in Japanese patent application laid-open No. 2-169853 and in Japanese patent application laid-open No. 63-97862 are well known.
For example, the device described in Japanese patent application laid-open No. 2-169853 is directed to diesel engine control which memorizes quantity of exhaust re-circulation at the time when an automobile running condition is hanged from normal running to the engine deceleration on idling state and continues to control the exhaust re-circulation quantity for the specified duration of time depending on the quantity of exhaust re-circulation at that time.
Another device described in Japanese patent application laid-open No. 63-97862 also relates to diesel engine control, which controls the quantity of exhaust re-circulation depending on degree of variation in the correction speed when the correction speed for controlling engine speed to the specified value during idling or low engine speed changes.
Thus, appropriate reduction in the amount of NOx in exhaust gas emissions is attained through use of the exhaust re-circulation control even when the engine is idling.
The internal combustion engines that conventional exhaust re-circulation control devices were designed for are diesel engines, and, although exhaust gas emissions control can be improved through use of conventional exhaust re-circulation control devices, the cleaning ratio of catalytic converters for gasoline engines has not been taken into account at all.
Therefore, even if the conventional re-circulation control devices are used for gasoline engines, optimal emissions cannot always be obtained. In particular, even if devices are adapted for gasoline engines like the one described in Japanese patent application laid-open No. 2-169853 which continue exhaust re-circulation control for a specified time after the engine switches to the idling state, HC emissions may adversely be increased. Also, when the gasoline engine is warming up, excessive re-circulation of exhaust gas emissions could occur, which could lead to deterioration of driveability and to engine stalling.